In recent years, a hierarchical virtual storage system has been known which uses high capacity and less expensive recording media such as magnetic tapes as a library device, and a recording medium accessible at a higher speed such as a hard disk drive (HDD) as a cache device. The virtual storage system is provided with a controller device configured to control accesses to the cache device and the library device, and the controller device causes a host device to virtually recognize data stored in the cache device as data of the library device. With such configuration, the host device can use a high-capacity storage region provided by the library device as if the region was connected to the host device itself (see Japanese Laid-open Patent Publication No. 2013-196318).
Meanwhile, a technique for making data redundant by using a parity has been known as a technique for averting loss of data stored in a storage system or the like. As an example of such technique, there is an apparatus in which multiple disk devices forming a parity group and a single storage device not included in the parity group are connected to the same channel, the apparatus configured to, during data transfer to the single storage device, read data restored by using data in the disk devices in the parity group and the parity (see Japanese Laid-open Patent Publication No. H09-218753). Further, there is a system configured to divide data into multiple data subsets (segments), generate parity data from the multiple data subsets, and regenerate one or more of the multiple data subsets by using the other data subsets and the generated parity data (see Japanese National Publication of International Patent Application No. 2012-501508).
Also, the following storage controller has been proposed which is configured to reduce data read time by using parity. When writing multiple data into a magnetic tape, the storage controller stores parity calculated based on the multiple data into a storage device accessible faster than the magnetic tape. Then, when reading the multiple data from the magnetic tape, the storage controller reads first data from the magnetic tape, and concurrently restores second data by using an already-read data region in the first data and the parity within the storage device (Japanese Laid-open Patent Publication No. 2013-196318).
However, the storage controller, which reduces the data read time by using parity, may not start restoration of the second data without a time interval after the start of reading the first data from the magnetic tape. As seen from the above, the later the restoration of the second data starts, the shorter the duration in which reading of the first data and restoration of the second data are performed concurrently. Consequently, there is a problem that it takes a long time to read all the desired data.